Extract method to check for a filesystem storage

We'll use this method to write a test dealing with remote storages.

app/models/concerns/attachable.rb

@@ -26,7 +26,7 @@ module Attachable
   end
 
   def set_cached_attachment_from_attachment
-    self.cached_attachment = if Paperclip::Attachment.default_options[:storage] == :filesystem
+    self.cached_attachment = if filesystem_storage?
                                attachment.path
                              else
                                attachment.url
@@ -34,7 +34,7 @@ module Attachable
   end
 
   def set_attachment_from_cached_attachment
-    if Paperclip::Attachment.default_options[:storage] == :filesystem
+    if filesystem_storage?
       File.open(cached_attachment) { |file| self.attachment = file }
     else
       self.attachment = URI.open(cached_attachment)
@@ -51,6 +51,10 @@ module Attachable
 
   private
 
+    def filesystem_storage?
+      Paperclip::Attachment.default_options[:storage] == :filesystem
+    end
+
     def validate_attachment_size
       if association_class && attachment_file_size > max_file_size.megabytes
         errors.add(:attachment, I18n.t("#{model_name.plural}.errors.messages.in_between",